MagicPen Marks best in slot, no doubt.
For seals Manaregen per level or flat
Glyphs I like to go flat magicresist instead of AP because it will help you in those 1v1 early game in mid. 13 magicresist more is way more useful than 9 AP or 28 AP lategame imo..
Quints flat AP best in slot. But flat HP are okay too